Late in the 1900s, Fabry and Perot learned of the potential of an interferometer formed by two parallel surfaces with highly reflective coatings and variable separation. By the 1970s, the basic Fabry-Perot interferometer had evolved to support many applications involving multiple beam interferometry, including applications with laser cavities and non-planar surfaces.
The Fabry-Perot interferometer is still widely used today. However, with increasing demands on performance, and with the miniaturization of optical systems, improvements are sought. One object of the invention is to therefore provide a Fabry-Perot interferometer that is small and finely tunable, as compared to the prior art. This object and others will be apparent in the description that follows.
The following patents provide useful background information for the invention: U.S. Pat. Nos. 6,040,944; 6,005,995; 5,917,626; 5,799,121; 5,739,945; 5,684,632; 5,682,452; 5,666,225; 5,642,448; 5,629,995; 5,612,824; 5,606,439; 5,481,402; 5,453,827; 5,361,155; 5,287,214; 5,283,845; 5,251,275; 5,212,745; 5,212,584; 5,073,004; 5,062,684; 5,039,201; 4,861,136; 4,813,756; 4,789,219; 4,550,975; 4,474,424; 4,466,699. Each of the afore-mentioned patents is expressly incorporated herein by reference.
The following articles and books provide useful background information for the invention, and are thus incorporated herein by reference: M. Hercher, The spherical mirror Fabry-Perot interferometer, Appl. Opt. 7, p. 951 (May 1968); K. Repasky et al., High finesse interferometers, Appl. Opt. 34, p. 2615 (May 1995); A. Siegman, Lasers, University Science Books, (1986); and The application of capacitance micrometry to the control of Fabry-Perot etalons, J. Phys. E 17, p. 48-54, (1984).
The invention of one aspect provides a miniature, fiber-coupled Fabry-Perot interferometer with a piezoelectric transducer and a collimating lens. As used herein, xe2x80x9clensxe2x80x9d generally refers to a plurality of lenses or optical components used to perform the function of collimating light energy across a gap as part of the interferometer. Specifically, in accord with this aspect, light energy is coupled to the collimating lens and the collimating lens forms a substantially collimated beam across the gap; the piezoelectric transducer expands or contracts to adjust the gap, thereby adjusting the free spectral range of the interferometer. Preferably, one or more lens surfaces forming the gap are coated as optically reflective surfaces (i.e., to form mirrors or partial mirrors); these surfaces preferably include those that face each other to form the gap. The surfaces may be planar or curved. Preferably, surfaces of the lens that do not form the optically resonant gap are coated with anti-reflective coatings.
In one aspect, the transducer is contoured with a shape facilitating the alignment and placement of the lens with the transducer. By way of example, in one aspect the contour shape of part of the transducer includes a V-groove, which facilitates both alignment and tunability, as described herein.
In another aspect, the lens of the invention utilizes gradient index (xe2x80x9cGRINxe2x80x9d) lenses. The GRIN lenses facilitate coupling to fiber optics, which provide the light energy for the interferometer, and further simplifies collimation within the interferometer. Preferably, highly reflective optical coatings are applied to the GRIN lenses to form the cavity across the gap. In a further aspect, a capacitive sensor is integrated with the interferometer to provide for absolute optical, and/or spatial, calibrations. GRIN lenses of the invention may for example include 0.75-pitch or 1.25-pitch.
As understood by those skilled in the art, the invention has several advantages and may be used within or in conjunction with any of the following devices or systems: tunable optical filters; tunable optical receivers; cascaded tunable optical filters; multi-pass tunable optical filters; an optical signal xe2x80x9cslicerxe2x80x9d for separating odd and even channels in WDM networks; a tunable wavelength division multiplexer; a tunable add/drop optical filter; a tracking filter for tunable lasers; an intra-cavity filter for tunable lasers; an amplified spontaneous emission noise suppression filter; and an optical spectrum analyzer in a selected waveband, e.g., visible or infrared. The invention may also be used with devices in the UV wavelength spectrum, for instance as an optical spectrum analyzer.
Specific advantages of the invention include its ease of construction, ease of optical and fiber alignment, use of non-specialized optical components (e.g., GRIN lenses), and its ease of optical alignment with low optical loss. These advantages simplify the interferometer, making it less expensive to manufacture as compared to prior art designs. The interferometer of the invention is also very flexible, providing for convenient adjustment of the free spectral range. With its few components, the interferometer of the invention is intrinsically small for coupling to fiber optics; accordingly it may be conveniently placed within a small hermetically sealed package, similar to telecom-grade optical devices.
In still another aspect, the GRIN lenses have optical curvature on the surfaces forming the gap, thereby forming an interferometer as a stable resonator, as known in the art. Such an interferometer provides for higher finesse. In one aspect, the GRIN surfaces forming optical collimation within the gap cavity are coated with one or more reflective coatings. Suitable GRIN lenses of the invention include those made by LightPath Technologies (xe2x80x9cGradiumxe2x80x9d), NSG America (xe2x80x9cSELFOCxe2x80x9d), or Casix.
In one aspect, one or more optical fibers are attached to the collimating lens; an input optical fiber brings optical energy to the interferometer and an output optical fiber carries processed optical energy from the interferometer to other devices or systems. In one aspect, the lens includes GRIN lenses that are pre-aligned by a commercial lens vendor, thereby avoiding later assembly.
In still another aspect, the collimating lens includes a deposition of metal (typically gold) on the outer, cylindrical surface (not on the optical surfaces forming collimation). The metal of this aspect creates an electrical capacitor. By way of example, one xe2x80x9cplatexe2x80x9d of the capacitor is the outer surface of the first GRIN lens, and the second xe2x80x9cplatexe2x80x9d of the capacitor is the outer surface of the second GRIN lens. The distance between the two GRIN lenses then also serves as the separation between the capacitor plates, as described herein. As the distance between the collimator lenses varies, so does the capacitance across the capacitor. This capacitance is a highly accurate measure of the absolute distance between lens surfaces, and thus provides an active calibration technique.
In an alternative aspect, a metal-cased package is used, rather than a metal deposition, to form the capacitor for calibration.
Yet another aspect of the invention involves a slight modification to the collimator lens discussed above. Specifically, the collimator lens (e.g., GRIN lenses) are more optically stable when formed with curved mirror surfaces (typically spherical surfaces) used in forming the gap. In accord with the invention, such spherical surfaces may be polished onto the collimator lens prior to optical coating. The resulting interferometer function simulates a xe2x80x9csymmetric, spherical mirrorxe2x80x9d resonator, rather than a xe2x80x9cflat-flatxe2x80x9d resonator, as known in the art. The advantage of using such a resonator is that higher values of finesse are achieved for the same optical coatings (i.e., the optical losses are effectively decreased), leading to increased resolution capabilities for the interferometer.
Another aspect of the invention includes a substantially matched pair of collimator lenses for use in the interferometer. In one aspect, for example, a xe2x80x9c0.5-pitchxe2x80x9d GRIN lens is used, with fiber coupling on each side. Such a lens transfers light from one fiber to the other; and optimum fiber-to-fiber coupling is achieved to minimize optical losses. Preferably, the 0.5-pitch GRIN lens is cleaved along its center to form two xe2x80x9cmatchedxe2x80x9d collimator lenses (i.e., two 0.25-pitch GRIN lenses). The matched lenses have smaller optical losses, generally, than two independently constructed collimators.
In one aspect, the piezoelectric transducer is a multi-layer stack piezoelectric element; in another aspect, the transducer is a monolithic piezoelectric element.
In the preferred aspect of the invention, the collimator lens rests in a V-groove formed in the transducer, which serves to xe2x80x9cself-alignxe2x80x9d the optical axis of the interferometer. By positioning the lens into the V-groove, the interferometer is effectively constructed and aligned in a single step.
In one aspect, the gap distance formed by the lens is coarsely adjustedxe2x80x94by coupling the lens with the transducerxe2x80x94to set the coarse free spectral range for the interferometer. Typical gap distances are between about 10 xcexcm to 10 mm. Fine adjustment of the gap is then made by applying a voltage across the transducer, thereby expanding or decreasing the gap to adjust the free spectral range. The lens is typically fixed into place with the transducer by a variety of techniques, e.g., by epoxy or mechanical means.
More particularly, when voltage is applied to the piezoelectric transducer, the transducer expands (or contracts) along its axis, such as along the axis of the V-groove, and the collimator lens also moves because it is fixed to the piezoelectric transducer. When the piezoelectric transducer expands, the gap distance increases, which varies the resonant wavelength inside the interferometer. If the piezoelectric transducer contracts, then the gap distance decreases. These expansion and contraction actions provide for optical tuning capability within the interferometer. A typical piezoelectric transducer of the invention expands or contracts by about 1-10 xcexcm, a distance sufficient to tune the interferometer over a desired wavelength range of about 1-10 optical wavelengths.
One advantage of using a piezoelectric transducer as the mechanism to adjust the gap, as compared to motors or temperature-based techniques, is that the tuning speed is very fast. Accordingly, in accord with the invention, the free spectral range may be obtained more quickly with the piezoelectric element as compared to other motion transducers.
In one aspect, the invention provides a miniature, fiber-coupled Fabry-Perot interferometer with a collimating lens and a wavefront retarder. In accord with this aspect, light energy is coupled to the collimating lens and the collimating lens forms a substantially collimated beam across the gap; the retarder is within the gap and has an optical path length that sets the free spectral range of interference within the gap. Preferably, a controller electrically controls the retarder to set the free spectral range. In certain aspects, the retarder takes the form of a liquid crystal or elecro-optic device.
In a preferred aspect, the invention incorporates passive temperature compensation, as known in the art, into transducer and lens structures to decrease mechanical and optical drifts, and to reduce and/or eliminate the need for a capacitance sensor. Additional optical elements may also be placed within the Fabry-Perot cavity to create additional functionality, as desired.
In accord with the invention, placement of the collimating lenses with the motion transducer sets the xe2x80x9ccoarsexe2x80x9d free spectral range of the interferometer. Generally, this coarse free spectral range is between about 1 nm and 100 nm. Once the coarse free spectral range is set, fine adjustment of the free spectral range is determined by control of an active element, such as the piezoelectric transducer. Generally, this fine adjustment of free spectral range covers between about one and ten wavelengths.
The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention.